Field of the Invention
This invention relates to a system of data recording, and in particular to a system for plotting a highly space-compressed train of electrocardiograph (ECG) waveforms, here termed a miniature ECG.
Systems for utilizing electrocardiograph signals for various display purposes are well known in the technology. Typically, with the subject in a resting state, data signals are received for a duration of only a few minutes and are then recorded on strip chart form for diagnosis. Other techniques employ long term heart monitoring and subsequent graphical recording of ECG signals for other diagnostic purposes. For example, long term monitoring allows a diagnostician to evaluate transient cardiac abnormalities which may be represented in the ECG of an ambulatory person as he conducts his daily business. The person will carry a tape recorder receiving ECG data from sensors affixed to the person. At a later point in time the tape is converted into a visible record and the ECG correlated to activity of the person. The amount of data accumulated is very large. U.S. Pat. No. 4,214,590 describes a technique of recording ECG signals onto tape and then transcribing that data onto hard copies in a compressed format. The recording per se utilizes a conventional endless loop with stylii controlled by galvanometers to perform the actual tracing. Thus, as shown in FIG. 3 of the '590 reference, tracing proceeds along an axis parallel to the direction of movement of the paper. The time axis is the direction of paper movement.
Techniques of data compression to eliminate redundant data in ECG systems is recognized in U.S. Pat. No. 4,090,505. As described therein, a reduction in redundant data is achieved while preserving random or infrequent occurring phenomena which may not normally be observed. Thus, variations in rhythm, that is arrhythmias which are detected by observing the resulting atypical waveforms or aberrations in the electrocardiogram tracing that are preserved. At the same time, the overall volume of electrocardiographic data is reduced in size such that a sufficient amount of routine data is preserved for comparison while at the same time all atypical data is displayed. Thus, by a technique of data compression which eliminates redundant data a medically acceptable electrocardiogram of greatly reduced size is achieved. The output occurs on a normal chart strip recorder having a standard resolution. That is, the dimension of the chart parallel to the direction of movement displays time relationships. Chart speed is in the range of 25 mm/sec with the stylus having the ability to denote of the order of .+-.2.5 millivolts of heart electrical activity. Consequently, the '505 reference utilizes a standard graphical output of electrocardiographic data, albeit eliminating that which appears to be redundant.
Other techniques of data compression, while not having specific application to ECG technology are known. For example, U.S. Pat. No. 4,109,243 describes a technique of storing data that would otherwise be lost during the retrace-blanking in a CRT scan. Data is plotted on a compressed time scale during the normal scan phase. The system therefore produces time compression by utilizing a recirculating memory storage for the temporary storage of input data followed by reading in a serial output, time compressed form. These techniques are not applicable to ECG plotting wherein continuous tracing takes place.
Other techniques deal with CRT plotting implementation where there is a difference between plotting resolution and the accuracy of the data base. U.S. Pat. No. 4,074,281 is representive of such technology employing line segment generation of multi-value time functions. Other references considered vis-a-vis the issue of CRT display or, generally the concept of data display systems, are represented in U.S. Pat. Nos. 3,680,076; 3,686,662; 3,812,491; and 3,902,476. None of those techniques are germane to data compression of ECG data.
While data collection and display technology has focused on the problem of accurate representation of voluminous, repetitious ECG data, there also exists a requirement to significantly simplify not only the data output but the equipment used to provide that pictorial record. Additionally, for some applications, high resolution is not necessary. For example, in the case of an ambulatory ECG system only gross variations in waveform need to be initially identified. Detailed analysis of the P, Q, R, S, T portions of a heart cycle waveform can take place after an arrhythmia has been identified. The use of precise ECG instruments for high resolution plotting is inconsistent with the initial use of ambulatory data. Such high resolution plotting is also costly and time consuming. Consequently, for this application, if all data is displayed in compressed format, arrhythmias are not lost in data compression. Thus, while low resolution data recording takes place for initial screening, the output is satisfactory. The problem to be addressed is both to reduce the volume of the printed record and to employ adequately precise (but much more rapid and less costly) plotting equipment compared to current high resolution ECG plotting methods.
Many existing diagnostic ECG systems rely on galvanometer technology utilizing a movable pen stylus and precise paper handling apparatus. Such are required, for example, as recognized in U.S. Pat. Nos. 4,090,505 and 4,184,487 for the purpose of obtaining an accurate standardized tracing. However, if accurate, high resolution is not required, alternative printhead technology, for example a thermal printhead utilizing a linear dot array, may be utilized. Such printheads are employed in some conventional computer output devices, and require a thermally sensitive recording medium. (Similar linear dot array printhead technology uses spark/arc "writing" via electroerosion or chemical action on an appropriately prepared recording medium.)
However, even if this equipment is used there is a requirement to provide for a significant amount of data compression to take place. For example, printing high resolution diagnostic electrocardiogram waveforms conventionally takes place using the direction of paper movement as the time axis. Thus, in applications where a low resolution is acceptable, i.e., ambulatory ECG's, significant reductions in paper output are still not achieved by merely replacing conventional galvanometer stylus recorders with an alternative printhead technology.
The principal advantage of the thermal printhead or equivalent linear array technology, when combined with appropriate data compression, is the very high speed with which, compared to galvanometer output, a large volume of ECG data can be graphically presented. One minute of diagnostic high resolution galvanometer recorded ECG data takes one minute to plot; with the invention here described, the time to plot a minute of compressed ECG data can be significantly less than one second.